As the Los Angeles Police Department moved to box in a group of protesters outside the city’s downtown federal building last weekend, several people suddenly rushed the line. Chunks of concrete flew through the air, recalls Martín Hoecker-Martinez. He says a woman then ran up and down the group of bystanders, exhorting others to join the attack. “Whether she recruited people, I don’t know,” says Mr. Hoecker-Martinez, a college physics professor who had been peacefully walking the Los...